This invention relates to a lithium manganese oxide spinel. More particularly, it relates to lithium manganese oxide suitable as a cathode active material of lithium ion secondary battery excellent in cycling behavior.
Lithium ion secondary batteries, which work as a battery by charging or discharging lithium ion between a cathode and an anode, show a high operating voltage and a high energy density and can suitably find applications to cellular phones, portable personal computers, video cameras or electric cars.
As a cathode active material for lithium ion secondary batteries, a layered complex oxide of Li1xe2x88x92xCoOz (0xe2x89xa6xxe2x89xa61) have already been popularly put into practice since it can generate 4-V level high voltage and have a high energy density. On the other hand, the starting material of Co is a poor resource and expensive. Hence, in consideration of the possibility of a demand for lithium ion secondary battery being greatly increased, stable supply of the starting material is feared for and, further, its price might soar. Thus, it has been considered to utilize a complex oxide of unexpensive Mn as a cathode active material capable of substituting Lijxe2x88x92xCoO2.
Of manganese complex oxides, cubic lithium manganese oxide spinels have been variously investigated as cathode material of lithium secondary battery since M. M. Tackeray reported that Li ion can be occluded and discharged (Research Bulletin, Vol. 18, pp461 to 462 (1989)). The cubic lithium manganese oxide spinel is generally represented by the chemical formula of LiMn2O4, and have a spinel-type crystal structure. A lithium secondary battery using this complex oxide as a cathode active material undergoes change in composition between LiMn2O4 (discharged state) and xcex-MnO2(charged state) by Li intercalation and deintercalation. In addition to the cubic lithium manganese oxides, there exist tetragonal and orthorhombic lithium manganese oxides which are in a distorted form of the cubic system complex oxides.
However, in comparison with the aforesaid Li1xe2x88x92xCoO2, the lithium manganese oxides involve a serious problem of capacity deterioration when charging and discharging are repeatedly conducted at a temperature as high as 50 to 60xc2x0 C. With respect to this problem, there have been proposed (1) to improve crystallinity of the lithium manganese oxide or (2) to substitute part of Mn by another metal element to stabilize crystal structure and depress capacity deterioration.
In particular, lithium manganese oxides partly substituted by another element according to the aforesaid technique (2) (hereinafter such lithium manganese complex oxide is sometimes referred to as xe2x80x9csubstituted lithium manganese oxidexe2x80x9d) show more stable crystal structure than the simple complex oxides (unsubstituted) comprising two of lithium and manganese and, as a result, can depress deterioration of capacity after repeated charging and discharging, thus the technique being extremely effective. However, with the increased demand for performance in recent years, such technique at present is still insufficient.
The present invention has been made with the situation in mind, and it provides a lithium manganese oxide suited for lithium secondary batteries, particularly substituted lithium manganese oxide. It also provides a lithium secondary battery showing excellent cycling behavior, and a cathode for use in the battery. It further provides a process for manufacturing the substituted lithium manganese oxide suited for lithium secondary batteries.
As a result of intensive investigations to attain the above-described objects, the inventors have found that substituted lithium manganese oxides acquire a structure slightly different from each other depending upon the processes for manufacturing them, as is different from the complex oxides simply comprising the two elements of lithium and manganese, and that the slight difference in structure leads to difference in performance. That is according to the findings of the inventors, the conventional common process of calcining starting materials comprising a lithium source and a maganese source in the air is not sufficient, and substituted lithium manganese oxides having excellent performance can be obtained by conducting calcination in two steps of calcination in an atmosphere of low oxygen concentration and calcination in an atmosphere of high oxygen concentration. The inventors have also found that such lithium manganese oxides providing good performance can be specified by 7Li-NMR, thus having completed the invention.
That is, the gist of the invention is a lithium manganese oxide spinel comprising lithium, manganese and a metal element other than lithium and manganese, which satisfies a composition condition of the following formula (A):
(xxe2x88x921)+y+z=2xe2x80x83xe2x80x83(A) 
wherein x, z and y respectively represent, in order, molar ratio of lithium, molar ratio of manganese and molar ratio of the other metal element, and x=1 to 1.5, y=0.005 to 0.5 and z=balance), and which shows the ratio to a main peak intensity at 5xc2x140 ppm to a main peak intensity at525xc2x140 ppm (I0ppm/I500ppm) each obtained by 7Li-NMR measurement according to the following measuring method, falling within the following formula (B):
I0ppm/I500ppmxe2x89xa60.65y+0.02xe2x80x83xe2x80x83(B) 
Measuring Method
A 4-mm Magic Angle Spinning (MAS) probe is utilized. xcfx80 pulse echo method is employed as a measuring pulse sequence.
Measuring conditions;
Resonance frequency; 155.43 MHz
Measuring range; 1 MHz
MAS spinning number; 15 kHz
Measuring temperature; 25xc2x0 C.
Chemical shift reference; 0.1 M aqueous solution of LiCl (0 ppm)
Measuring pulse sequence; xcfx80 pulse echo method*1
Width of measuring pulse; 1 xcexcs for 90, 2 xcexcs for 180 degrees
Delay time xcfx80; 67 xcexcs (reciprocal of mAS spinning number of 15 kHz)
Repeating period; 64 sec
*1: 90-degree pulsexe2x80x94waiting time xcfx80-180-degree (xcfx80) pulsexe2x80x94waiting time xcfx80xe2x80x94observation
The thus obtained NMR spectrum is subjected to peak dividing in the following manner to obtain final peak strength. Peak-dividing conditions:
Fitting function; Lorentz type
Baseline correction; offset
Others; Fitting is conducted with the assumption that there is one main peak at each of 5xc2x140 ppm and 525xc2x140 ppm, treating others as side bands.
Additionally, as to the processes for manufacturing lithium manganese oxides, there are known literatures as follows.
Japanese Patent Laid-Open No. 245106/1995 discloses a process of manufacturing spinel structure of LixMn2O4 having a large specific surface area of 2 m2/g or more by calcining MnO2 and LiNO3 in a nitrogen gas at a low temperature (about 500xc2x0 C). It is described in this publication that use of crystalline spinel structure of LixMn2O4 having a large specific surface area serves to improve discharge capacity and cycling behavior at a high discharging rate.
In addition, it is proposed in Japanese Patent Laid-Open No. 247497/1998 to prepare a gel from a lithium salt, a manganese salt and a solution of polyvinyl butyral (chelating agent) and calcine the gel at 200 to 900xc2x0 C for 5 to 30 hours under an inert gas or under air, thus producing LixMn2O4 to be used as a cathode active material for lithium secondary battery.
However, such prior art does not suggest the invention at all.